OK I'll take a stab at answering your questions…
Does it hurt having any nofollow links anywhere on my blog?
No it does not hurt at all to have nofollow links on your blog. It is actually the opposite. A nofollow link means you are not passing PR to that site, meaning you have more PR to spread to other pages on your blog. Don't worry about mixing in some dofollow links to blogs you want to give credit to though.
Which one helps your pagerank the most; external or internal links?
It is external links point to your site that helps your pagerank the most. The internal links just spread existing PR to your other pages.
Do I have any control over external links?
Some external links you have no control over, but you have plenty of control over things like comments on other blogs and carnival submissions. You have the power to network with other bloggers in hopes of getting some links.
How do I get more quality dofollow external links to my blog?
See the answer above. First of all, the better quality your blog is, the more likely people will link to it. Just waiting for the links to happen takes too long though. Get out there and be active on other blogs to build some connections. You can also help spark connections via links on your own blog or social media shares. Of course e-mail is still the most powerful way to connect with other bloggers. Guest posts are another way to get quality links. If in doubt, check where other blogs get their links.